Probably they didn't purchased a license for a codec and didn't incorporated it into processor or do not want to use 3rd party one. IMHO it's licensing fees stopping them (and R&D fees too). When it will be a must for all Olympus cameras to have AVCHD, they will license one and you'll have to pay more for the camera with AVCHD.

Sure it's time to get rid of MJPEG and AVI and use MPEG4 H.264 with AAC for the videos. MP4 or MKV container.